How can I automate circuit on/off based on a voltage threshold in another circuit?

I hope you aren't trying to make batteries charge each other

You don't tell us enough.

Do the three batteries have a common 0V connection or are they fully isolated.

What is the current.

Do you need make before break or break before make switching.

What kind of batteries, are there any safety issues.

Best bet is explain what you are trying to do overall and fill in the details. A block diagram would help.

My first thought is that an Arduino or similar would be the ideal control element - because you'll find that you need hystereisis, timeouts and other features. You will still need power switches, relays are much easier to manage than MOSFETs.

Doing it with pure analogue is always possible but much harder to modify and tune.

If it's a serious commercial application you could use PLC type controllers - much more expensive than Arduino, and no easier to learn.

MK

MK,

Thank you for your response.  Yeah, I was pretty vague with my question.  Let me explain.  This is not a commercial application.  I am a theoretical physicist and I'm doing some experiments to test the properties of electricity.  So I want to set up three circuits that will each, in turn, run a small load like a computer fan and I will be adding different components to different circuits and taking measurements from each separate circuit to compare.  I haven't set this up yet.  I'm just in the design phase.

I would probably use lead-acid rechargeable batteries and have them totally separate. I just want to automate the on off so that only one circuit at a time is running the load.

You said you hope I'm not charging batteries with other batteries?  Is there a problem with that? That may be something I would experiment with in the future.  Are there safety issues with doing that?  Right now I just want to run a small load with one circuit at a time but switch them automatically because each circuit would be running the same load but only one at a time.

I will see if I can write up a more detailed question and give you more information, maybe upload a diagram too.

Thanks!

Scot

I'll look forward to seeing more detail about your plan.

MK

This wouldn't be that hard to design and there are many ways to do this.

1. An Arduino may be the easiest. The coding would be fairly simple. But the Arduino is for the control. You still need MOSFETS or BJT transistors with proper gate or base drive to do the actual switching. The details of how much current and voltage you're switching would need to be known to select the proper components.

2. An FPGA would also be a good choice, but again, there is some programming. You need either an FPGA with analog comparators or use a quad comparator in front of the FPGA.

3. Discrete gates and comparators would work as well, but there would be more breadboarding perhaps.

Make a diagram with voltages and current loads and we can then talk specifics.